Edible oils and fats such as but not limited to cocoa butter, when obtained by expelling and/or solvent extraction, often require extensive purification. Accordingly, they can be degummed, neutralised, bleached and/or deodorised. During the deodorisation treatment, the oil to be deodorised is first of all deaerated, then heated under vacuum to a deodorisation temperature and then sparged with a stripping medium which is usually steam. After sufficient stripping medium has been passed through the oil, it is cooled by heat exchange, first with incoming oil and then with cooling water to yield fully refined oil.
As a result of a low pressure (about 2-8 hPa), an elevated temperature (about 140-270° C.) and the use of a stripping medium such as steam, the most volatile constituents of the oil to be deodorised enter the gas stream being passed through the oil and are thus effectively removed from the oil. These volatile constituents can be malodorous compounds but can also be free fatty acids still present in the oil. In the latter case, the deodorisation process is commonly referred to as a physical refining or steam refining process.
The low pressure inside the deodoriser is usually maintained by the use of steam ejectors. Accordingly, the deodoriser is connected via a vacuum duct to the inlet of a booster pump or two booster pumps in series. High pressure motive steam is also supplied to this booster pump with the result that the vapour stream leaving the booster pump comprises stripping steam, volatile constituents stripped from the oil, some non-condensable gases and motive steam.
Formerly, it was not uncommon to pass this vapour stream to a direct condenser wherein the stream is treated with water as a result of which the volatile constituents stripped from the oil condense together with most of the water vapour contained in this vapour stream. The condenser is connected via barometric legs to a hotwell where the condensate is separated into an organic upper layer and an aqueous lower layer. The organic upper layer is then collected by decantation and the aqueous lower layer is re-circulated via a cooling tower. This process has been described for instance by A. J. C. Andersen, in ‘Refining of oils and fats’, second edition, Pergamon Press, 1962, pages 187-198. Because of the smells released by this cooling tower and its tendency to foul, modern systems prefer to scrub the vapour stream leaving the deodoriser before condensing most of the water contained in this stream in the condenser of the steam ejector battery (see for instance “Introduction to fats and oils technology”, second edition (2000), edited by R. D. O'Brien, W. E. Farr and P. J. Wan, AOCS Press, Champaign, pages 256-258). This scrubbing process entails passing said vapour stream through a scrubber device or scrubbing vessel in which an intimate contact is established between this vapour stream and the cooled condensate that is being circulated over said scrubber device or scrubbing vessel. The intimate contact between the vapour stream and the liquid condensate is commonly established by spraying the condensate into the vapour stream or by the use of a column packing. As a result, the least volatile constituents of this vapour stream condense onto said cooled condensate and, in doing so, increase its volume and raise its temperature. Accordingly, the condensate circulation system will comprise an indirect heat exchanger to control the condensate temperature and a bleed valve to control the amount of condensate in circulation, whereby this bleed valve will be connected to an intermediate storage for the deodorisate or fatty acid distillate.
In order to attain the highest degree of condensation of the least volatile constituents present in the vapour stream leaving the deodoriser, the temperature of the condensate circulating over the scrubber device or scrubbing vessel is preferably maintained just above the melting point of the condensate. In industrial practice this may, for example, be about 65-70° C. when the edible oil to be treated is cocoa butter.
In US-A1-2003/0097842, an apparatus for condensing a water-containing fluid is described. In this document, vapours evacuated from a deodoriser are passed over one of two low-temperature surface condensers, this system being also referred to as “dry condensing”. In this dry condensing system, the condenser that is in operation is kept at for instance −28° C., so that not only the least volatile constituents in said vapours will condense but also the steam used as stripping medium and as motive ejector steam. As a result, the condenser that is in operation will gradually be filled with solid condensate. The condenser that is not in operation is thawed in order to release its condensate and the vapour stream is switched from the one condenser to the other at regular intervals.
The operation of the above described systems relies upon the condensate being fully liquefiable so that it can be pumped in the scrubber systems and drains away when the dry condensers are thawed. However, when certain oils such as, but not limited to, cocoa butter are deodorised, the resulting condensate may contain one or more high melting constituents that prevent the condensate from melting completely under the operational conditions disclosed above. These high melting constituents may also be reasonably volatile so that they can be stripped out of the oil and then cause problems in the scrubber. Although, as illustrated hereinafter, this problem is of major concern for cocoa butter, it may also to some extent happen with illipe butter (used as cocoa butter equivalent component), sesame oil (due to the presence of about 1% sesamin, having a molecular weight of 354, melting at 123° C. and boiling at 235° C. under a pressure of 0.01 mm Hg), lanolin (due to the presence of a number of polyterpene alcohols such as lanol melting at 141° C. and agnol melting at 164° C.) and shea butter (due to the presence of β-amyrin having a molecular weight of 427, melting at about 187° C. and boiling at 260° C. under a pressure of 0.8 mm Hg, and the presence of lupeol having a molecular weight of 427 and melting at 215° C.). The foregoing is provided only as an illustration of what is meant herein as “high melting constituents” with respect to the present invention.
In the production of cocoa butter, cocoa beans, which have already been fermented after having been harvested, are first of all broken so that the shells can be separated from the kernels by winnowing. Then the cocoa kernels are treated with a concentrated alkaline solution, for example potassium carbonate, which treatment generates the typical chocolate flavour. Subsequently, the cocoa kernels or nibs are roasted and then ground in order to form cocoa liquor. Pressing this cocoa liquor yields both cocoa butter and cocoa powder.
World-wide about one third of the annual cocoa bean harvest of some three million tonnes is incorporated in chocolate as cocoa liquor. The remaining two thirds are separated into butter, which is then used in the manufacture of chocolate, and powder. Cocoa butter may be produced in three ways, i.e. by pressing, by screw expelling and by solvent extraction. Depending upon the way of producing, three grades of cocoa butter may be distinguished.
Prime pressed cocoa butter may be filtered, degummed with water or dilute citric acid and deodorised without loosing its denomination. Alkali refining and bleaching are only permitted for the lower grades of cocoa butter. Most chocolate manufacturers want to use a cocoa butter with an almost neutral taste. Therefore, deodorisation during the manufacturing process of cocoa butter is quite common. This means that annually some 800,000 tons of cocoa butter are deodorised, especially now that the pressing and expelling operations are moving towards the countries of origin.
According to B. W. Minifie in ‘Chocolate, cocoa and confectionery, science and technology’, AVI publishing Company Inc. Westport Conn., second edition, 1980, cocoa kernels or nibs contain about 1.5 weight % theobromine and 0.15 weight % caffeine. Because theobromine is only poorly soluble in fat, most of the theobromine remains in the powder on pressing. According to the ‘Sigma Product Information Sheet’, which has been reproduced below as table 1, cocoa powder may contain as much as 2.6 weight % theobromine whereas cocoa butter may contain as little as 0.01 weight % theobromine. On the other hand, the caffeine content of the crude cocoa butter is not that drastically reduced in comparison with the caffeine content of the cocoa nibs.
As shown in table 1, theobromine and caffeine have quite low molecular weights of 180 and 194 respectively so that they are quite volatile and sublime at atmospheric pressure at 290-295° C. and 178° C. respectively. As exemplified by the low theobromine and caffeine content of commercial cocoa butter, the deodorisation process almost completely removes these alkaloids from the cocoa butter. Consequently, the deodorisation distillate may exhibit a substantial alkaloid content.
TABLE 1TheobromineCaffeineMolecular formulaC7H8N4O2C8H10N4O2Molecular weight180.17194.20Melting point (° C.)357235-238Boiling point (° C.)290-295178 (sublimes)(sublimes)pH (1% solution)N.A.6.9pKa9.914.0 at 25° C.Specific densityN.A.1.2AppearanceWhite powderOdourless whitepowder orcrystals witha bitter tasteSolubility (1 g/ml) in:water at 20° C.200060water at 100° C.1501.5 (solubilityincreases byadding a dilutedacid)ethanol222066Strong alkalisforms stabledecomposedcompoundAmount (%) incocoa beans0.88-4.230.062-0.416(average0.214)cocoa powder2.60.1-0.5cocoa butter0.0080.038milk chocolate0.1-0.50.021dark chocolate10.17
Given the melting points of alkaloids such as theobromine and caffeine, it is not surprising that the condensate originating from cocoa butter deodorisation easily solidifies and thereby causes all kinds of deposits and blockages in the deodorisation equipment and especially in its scrubber section. These deposits are very difficult to remove. The alkaloids hardly dissolve in a non-polar solvent and when a polar solvent is used, the oily constituents of the deposits prevent this solvent from reaching the alkaloids. Consequently, manual cleaning using a hammer and chisel is often the only way to get rid of these deposits.